1. Field of the Invention
The present invention relates to a drive device for a vehicle provided with an engine that is automatically shut down under a predetermined shutdown condition and is automatically restarted under a predetermined restart condition.
2. Description of the Related Art
Idling stop vehicles that automatically shut down the engine when they come to a stop have recently been developed in order to reduce the amount of fuel that the engine consumes. These idling stop vehicles are configured to automatically shut down the engine when a predetermined shutdown condition is met, while they are configured to automatically restart the engine when a predetermined restart condition is met. Also, hybrid vehicles equipped with an engine and an electric motor are usually configured to automatically shut down the engine when they come to a stop.
Automatic transmissions provided with a plurality of hydraulic clutches and hydraulic brakes (hereinafter referred to as hydraulic clutches, including hydraulic brakes) are mounted in vehicle power trains. In order to deliver clutch control oil pressure to the automatic transmission, an engine-driven oil pump is connected to the automatic transmission. However, in vehicles equipped with an idling stop feature, the engine and the oil pump are shut down when the vehicle comes to a stop, causing the hydraulic clutch in an engaged state to become disengaged without keeping the engaged state. When the oil pump begins to operate again after the engine is restarted, the disengaged hydraulic clutch becomes engaged, whereby the hydraulic clutch produces an engagement shock.
In order to avoid the disengagement of the hydraulic clutch during an idling stop mode, an electric oil pump or accumulator may be used to prevent oil pressure from dropping during an engine shutdown. The installation of the electric oil pump, however, causes an increase in cost of vehicles equipped with an idling stop feature. Thus, a power transmission device having a spring incorporated in the piston of the hydraulic clutch has been developed to ensure that the hydraulic clutch is not completely disengaged even if the hydraulic clutch suffers from a reduced control oil pressure (for example, Japanese Unexamined Patent Application Publication No. 2006-9973).
As described above, even if the oil pump is shut down, the hydraulic clutch can be prevented from being completely disengaged by biasing the hydraulic clutch piston in an engagement direction with a spring. Doing this prevents the hydraulic clutch from producing an engagement shock at the time of an engine restart.
However, the vehicle transmission device disclosed in Japanese Unexamined Patent Application Publication No. 2006-9973 is configured to have a hydraulic clutch piston urged toward the engagement direction with a spring. Due to this configuration, the hydraulic clutch has a cancellation oil chamber that supplies hydraulic oil for its disengagement. In other words, in order to completely disengage the hydraulic clutch, hydraulic oil must be delivered to the cancellation oil chamber. For this reason, during an engine shutdown that causes the oil pump to be deactivated, hydraulic oil cannot be delivered to the cancellation oil chamber of the hydraulic clutch. As a result, the hydraulic clutch is held in a slipping or engaged state, whereby the vehicle cannot be switched to a neutral state.